1. Field of the Invention
The present invention relates generally to an apparatus for detecting occurrence of a misfire event in an internal combustion engine on the basis of an angular position signal which is obtained from a sensing blade assembly rotating together with a crank shaft through the medium of a crank angle sensor. More particularly, the invention is concerned with a misfire detecting apparatus for an internal combustion engine (hereinafter also referred to as the engine for short) which is capable of determining occurrence of misfire with high accuracy without being affected by error components contained in the angular position signal.
2. Description of Related Art
As the hitherto known misfire detecting apparatus for the internal combustion engine, there may be mentioned, for example, a technique disclosed in Japanese Unexamined Patent Application Publication No. 194346/1992 (JP-A-4-194346), according to which a period of a pulse signal generated at every unit crank angle is measured, wherein an index indicating variation of the period as determined on the basis of the period of the pulse signal (i.e., a value determined by differentiating three times a mean angular acceleration per unit crank angle) is compared with a predetermined reference value for deciding definitely occurrence of misfire in the engine. This known technique will be reviewed below in more detail for having better understanding of the background techniques of the present invention.
FIG. 8 is a schematic diagram showing a structure of the misfire detecting apparatus described above and incorporated in a control system for an internal combustion engine. Referring to the figure, an internal combustion engine 1 includes a plurality of cylinders (only one of which is shown representatively). A sensing blade assembly 2 (constituting part of reference angular position setting means) is mounted on a crank shaft of the engine 1. A crank angle sensor 3 detects the edges of segments of the sensing blade assembly 2 and outputs an angular position signal indicative of a corresponding reference angular position of the crank shaft.
A misfire detecting means 4 is designed to detect variation in the rotation of the engine 1 on the basis of the angular position signal SGT generated by the crank angle sensor 3 to thereby make decision as to occurrence of misfire. To this end, the misfire detecting means 4 is equipped with an input terminal 21 for fetching the angular position signal SGT, an output terminal 22 for outputting an abnormality signal E when occurrence of misfire is decided, an input terminal 23 for fetching a cylinder identifying signal SGC from a sensor means (not shown) mounted on a cam shaft of the engine 1, and an input terminal 24 for fetching a signal indicative of engine operation state D from a variety of sensors (not shown). A display unit 5 responds to the abnormality signal E for alarming a driver of occurrence of misfire when the misfire detecting means 4 detects the misfire at a ratio greater than a predetermined value.
FIG. 9 is a timing chart for illustrating the general operation of the conventional misfire detecting apparatus shown in FIG. 8 on the assumption that the misfire takes place only once in the course of normal combustion cycles. In FIG. 9, a reference symbol .omega. represents an actual angular speed of the engine 1, and .alpha. represents an angular acceleration which is arithmetically determined or calculated on the basis of a period or duration T between falling edges of the angular position signals SGT delivered from the crank angle sensor 3. Further, a reference symbol .DELTA..alpha. represents an angular acceleration deviation or difference between the preceding calculation value .alpha..sub.i-1 of the angular acceleration .alpha. and the current calculation value .alpha..sub.i, a symbol .DELTA..beta. represents a secondary deviation or difference between the preceding calculation value .DELTA..sigma..sub.i-1 of the angular acceleration deviation .DELTA..alpha. and the current calculation value .DELTA..alpha..sub.i thereof, and a symbol .DELTA..gamma. represents a tertiary deviation or difference between the preceding calculation value .DELTA..beta..sub.i-1 of the secondary deviation .DELTA..beta. and the current calculation value .DELTA..beta..sub.i thereof.
As shown in FIG. 9, there are determined decision reference values C.sub.i, C.sub.i-1 and C.sub.i-2 (see broken lines) for the current value .DELTA..gamma..sub.i, the preceding value .DELTA..gamma..sub.i-1 and the prepreceding preceding value .DELTA..gamma..sub.i-2 of the tertiary deviation .DELTA..gamma., respectively. The misfire detecting means 4 is designed to compare the values .DELTA..gamma..sub.i, .DELTA..gamma..sub.i-1 and .DELTA..gamma..sub.i-2 with the reference values C.sub.i, C.sub.i-1, and C.sub.i-2, respectively, to thereby decide occurrence of misfire when the conditions mentioned below are simultaneously satisfied: EQU .DELTA..gamma..sub.i &gt;C.sub.i EQU .DELTA..gamma..sub.i-1 &gt;C.sub.i-1 EQU .DELTA..gamma..sub.i-2 &gt;C.sub.i-2
Next, referring to FIGS. 10 and 11, description will be made of error involved in the misfire decision processing executed by the conventional misfire detecting apparatus shown in FIGS. 8 and 9.
In general, the crank angle indicated by the edge of the segment of the sensing blade assembly 2 contains some error because of unavoidable manufacturing tolerance (i.e., limitation imposed on the precision with which the sensing blade assembly 2 is to be manufactured).
FIG. 10 is an enlarged side elevational view showing schematically a structure of the sensing blade assembly 2. Referring to the figure, the sensing blade assembly 2 is comprised of a center portion or cylindrical body 11 and segments 12 and 13 formed of a same material with the cylindrical body 11 in an integrated structure. The sensing blade assembly 2 is mounted on the crank shaft 14 of the engine 1.
In the sensing blade assembly, the one segment 12 serves to determine the timings of the angular position signals SGT corresponding to the angular position located before the top dead center by 75.degree. (hereinafter represented by BTDC 75.degree. ) and the angle position before the top dead center by 5.degree. (hereinafter represented by BTDC 5.degree. ) for each of the first and fourth cylinders #1 and #4, respectively, while the other segment 13 serves to determine the timings of the angular position signals SGT corresponding to BTDC 75.degree. and BTDC 5.degree. for the second and third cylinders, respectively.
The angular position signal SGT generated by the crank angle sensor 3 contains unavoidable error components due to a number of causes or factors. By way of example, there may mentioned as these factors an inter-edge angle error .epsilon.1 in the angular extensions or spans of the segments 12 and 13 which is involved in manufacturing the sensing blade assembly 2 as a unitary structure and an eccentricity error .alpha.2 in the center of rotation of the cylindrical body 11 as involved in mounting the sensing blade assembly 2 on the crank shaft 14. Thus, the angular position signal SGT contains error components affected significantly by these error factors .epsilon.1 and .epsilon.2.
FIG. 11 is a timing chart for illustrating variations in the quantities .alpha.,.DELTA..alpha., .DELTA..beta. and .DELTA..gamma. as determined by simulation on the assumption that the inter-edge angle error contained in the angular position signal SGT due to the eccentricity error .epsilon.2 is 0.4.degree.. As can be seen from FIG. 11, when the misfire detection processing is executed by using the tertiary deviation .DELTA..gamma. in accordance with the decision condition mentioned previously, there arises a situation in which the misfire decision conditions are not satisfied for a misfire event because the tertiary deviation contains detection error of 0.4.degree.. More specifically, because .DELTA..gamma..sub.i &gt;C.sub.i, .DELTA..gamma..sub.i-1 &lt;C.sub.i-1, and .DELTA..gamma..sub.i-2 &gt;C.sub.i-2 in this case, the aforementioned conditions for deciding occurrence of misfire are not met. In this manner, the conventional misfire detecting apparatus suffers a problem that the occurrence of misfire can not be detected with reliability because of incapability of performing the misfire decision processing with accuracy. Of course, in the conventional misfire detecting apparatus, there may arise such a situation in which occurrence of misfire is erroneously detected even when variation in the engine speed (rpm) due to the misfire event does not take place in actuality.
As is apparent from the above, in the misfire detecting apparatus of the engine known heretofore, decision as to occurrence of misfire is realized by comparing the pre-preceding value .DELTA..gamma..sub.i-2, the preceding value .DELTA..gamma..sub.i-1 and the current value .DELTA..gamma..sub.i of the tertiary deviation .gamma..gamma. of the mean angular acceleration .alpha. with the decision reference values C.sub.i-2, C.sub.i-1, C.sub.i, respectively. Accordingly, when the value of .DELTA..gamma. suffers error contained in the angular position signal SGT, the misfire detecting apparatus known heretofore can not detect the occurrence of misfire with reliability.